Warming of Oceans Will Reduce and Rearrange Marine Life

The warmth of the ocean is the critical factor that determines how much productivity and biodiversity there is in the ocean, and where.

In two separate studies, researchers found that warming oceans have led to a massive decline in the amount of plant life in the sea over the last century, and that temperature is tightly linked to global patterns of marine biodiversity.

“We are just now understanding how deeply temperature affects ocean life,” said biologist Boris Worm of Dalhousie University, a co-author on both papers appearing July 28 in Nature. “It is not necessarily that increased temperature is destroying biodiversity, but we do know that a warmer ocean will look very different.”

In one study that looks at historical records of algae abundance over the last hundred years, Worm and his co-authors found that warming ocean temperatures are correlated to a massive decline in the amount of marine algae, or phytoplankton. Marine algae are the base of the entire ocean food chain, and were also responsible for originally creating oxygen on the planet.

The study estimates the decline in marine algae has been approximately 40 percent since 1950.

“I think that if this study holds up, it will be one of the biggest biological changes in recent times simply because of its scale,” said Worm. “The ocean is two-thirds of the earth’s surface area, and because of the depth dimension it is probably 80 to 90 percent of the biosphere. Even the deep sea depends on phytoplankton production that rains down. On land, by contrast, there is only a very thin layer of production.”

The study on marine phytoplankton is the first to look at changes over the last century at a global scale with data from as far back as 1899. Similar models have been made using satellite data, but that data only extends back to 1979.

“One of the most important aspects of the new paper is that they’ve come up with the same answer but from a different approach than we saw from space,” said marine botanist Michael Behrenfeld of Oregon State University. “I think that we should be concerned that this convergence of multiple approaches sees a reduction in the phytoplankton pigments as the ocean warms. If we continue to warm the climate we will probably see further reductions.”

In a study of general marine biodiversity, scientists have made the first global map of the biodiversity of the oceans for more than 11,000 marine species, from tiny shrimp-like creatures to whales, building on 6.5 million records from the Census for Marine Life and other databases. Of all the factors they looked at to explain why some regions had more or fewer types of creatures, the only factor that consistently explained the patterns for the 13 groups of marine life they studied was temperature.

“It was surprising that we found such a strong correlation to marine biodiversity and temperature,” said biologist Derek Tittensor of the University of Dalhousie, lead author of the marine biodiversity map study. “You might expect a different response to temperature from cold and warm-blooded animals, for example.”

Ocean temperature had different effects on the number of different creatures in coastal habitats versus open-ocean habitats. The biodiversity hotspots for coastal marine ecosystems were mostly near the equator where ocean temperatures are warmest, much like on land.

But for open ocean ecosystems, which included many deep-sea creatures, whales and big fish like tuna, the hotspots for diversity were at the mid-latitudes, where temperatures were slightly cooler.

“What we can draw from this study is that it is very likely that we will see a reorganization of biodiversity in the ocean from a warming ocean, but right now it’s very hard to predict exactly what that reorganization will be,” said Tittensor.

The hotspots in biodiversity are also the areas that have attracted the most human impacts, such as fishing and habitat destruction, meaning that we are harming the areas that we should be trying to conserve.

By mapping where the biodiversity of marine life is today, scientists now have a baseline for comparing species distributions in the future. Understanding these changes will help them understand how marine biodiversity is being affected by changes in the amount of marine algae, for example.

“In order to understand life in the ocean, we need to understand where it is,” said Worm. “It’s a basis for understanding and also managing ocean life.”

“The ocean is something that we’re not very good at thinking about,” Worm added. “It is one of those things that is so big to see that it has been hard to see it until now.”